Ann E. Blythe

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite-in, staurolite-in, garnet-in, and chloritoid-in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E-W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N-S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post-D1 and pre- to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end-member metamorphic core complex-type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0‐1.8 mm/yr) than inside of it in the east (0.55‐0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region’s recent tectonic evolution.

Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite (U-Th)/He analyses

Received 5 May 2006; revised 19 October 2006; accepted 22 December 2006; published 4 May 2007. [1] New apatite and zircon fission track and (U-Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7z ircon fission track (ZFT), and 7a patite (U-Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie � 2 km above the adjacent valleys, and a transect of >20 valley bottom samples spans from the Lesser Himalaya across the GreaterHimalayaandintotheTethyanstrata.Asaconsequence, these data provide one of the more comprehensive low-temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFTages range from >3.8 to 0 Ma, ZFTages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the southcentralzoneofthefieldarea)yieldedsignificantly older ZFTand AFTagesof � 2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFTand AFTages from a vertical transect along this ridge indicate exhumation rates of � 1.5 km Myr � 1 (r 2 > 0.7) from � 2 to 0.6– 0.8 Ma, whereas AHe ages indicate a faster exhumation rate of � 2.6 km Myr � 1 (r 2 = 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr � 1 (all with low r 2 values of <0.6) for the time period from � 1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long-term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than intheGreater Himalaya andmay bearesponse toa drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data arefurtherevaluatedwiththermokinematic modelingin the companion paper by Whipp et al. Citation: Blythe, A. E., D. W. Burbank, A. Carter, K. Schmidt, and J. Putkonen (2007), Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U-Th]/He analyses, Tectonics, 26, TC3002, doi:10.1029/2006TC001990.

Tectonically versus climatically driven Cenozoic exhumation of the Eurasian plate margin, Svalbard: Fission track analyses

Apatite and zircon fission track (FT) thermochronology, as well as regional geologic data, are used to assess the tectonic and climatic mechanisms responsible for exhumation of Svalbard during the last 70 million years. Four stratigraphic profiles in the Paleogene Central basin spanning ∼0.6–1.1 km vertical relief yield apatite FT ages ranging from 28.3 ± 2.3 Ma to 41.8 ± 3.7 Ma, with a mean age of ∼36 Ma and no apparent vertical trend. Apatite ages from the Paleogene Forlandsundet and Kongsfjorden basins and the Late Cretaceous-Tertiary Svalbard Orogen range from 26.8 ± 3.7 to 55.7 ± 4.7 Ma, with Kongsfjorden data suggesting cooling earlier than the Central basin. Although 10 detrital zircon samples were analyzed from both the Forlandsundet and Central basins, only one sample (from the basal Paleogene deposits in the northwesternmost Forlandsundet basin) had a reset age of 42.4 ± 1.8 Ma. Nonreset detrital zircons elsewhere retain Caledonian and Proterozoic detrital ages. Thermal-history models derived from apatite track-length distributions of six Central and Kongsfjorden basin samples indicate a strikingly consistent five-phase thermal history. Initial uplift of sediment sources to the north and west is recorded as a 70–50 Ma cooling signature. Following deposition from 63 to 49 Ma, the samples were heated until ∼35 Ma, probably as the result of continued sediment burial. This basin filling coincided with a dextral continental transform connecting spreading ridges in the Arctic and Norwegian-Greenland oceanic basins. The thermal models are consistent with ∼40°–50°C of cooling from ∼35 to 25 Ma. This cooling, which is consistent with deposition patterns both onshore and offshore, appears to be the result of rift-related uplift and erosion as the Atlantic spreading ridge propagated northward to separate Greenland from Svalbard. Track-length models are consistent with thermal stasis from ∼25 Ma to 5 Ma, despite sedimentation in offshore Miocene basins suggesting that continued erosion was occurring. We suggest that an increase in the geothermal gradient (by the “Yermak Hot Spot” and associated onshore volcanism) offset cooling expected at this time as the result of continued erosion. A final rapid post-5 Ma cooling phase of ∼70°C is attributed to ∼2.1 km of erosion in response to intensified glacial denudation. Pliocene-Holocene cooling, which occurred despite a persistently elevated geothermal gradient, is consistent with offshore evidence for first ice cover in the Norwegian-Greenland Sea at 5.5 Ma and a substantially increased sediment flux to continental-slope basins from 5.0 to 0.44 Ma.

No frictional heat along the San Gabriel fault, California: Evidence from fission-track thermochronology

Large earthquakes generate frictional heat, and the magnitude of heating is related to the slip magnitude, the applied effective normal stress, and the frictional strength of the fault. We looked for evidence of this heating in apatite fission-track age and track-length distributions of samples from adjacent to and within the San Gabriel fault zone in southern California. The fault is thought to be an abandoned major trace of the San Andreas fault system active from 13 to 4 Ma and has since been exhumed from depths of 2-5 km. At our sample locality, as much as 40 km of total slip is thought to have accumulated along a localized ultracataclasite layer just 1-8 cm thick. We see no evidence of a localized thermal anomaly in either fission-track ages or track lengths-even in samples within just 2 cm of the ultracataclasite. Because of the absence of any measurable impact on fission tracks, we have been able to use forward modeling of heat generation, heat transport, and fission-track annealing to constrain the frictional properties of the fault. We find that either there has never been an earthquake with >4 m of slip at this locality, or the average apparent coefficient of friction must have been <0.4.

Controls on the erosion and geomorphic evolution of the San Bernardino and San Gabriel Mountains, southern California

We have investigated what controls geomorphic evolution of active mountain belts by comparing the patterns of erosion in the San Gabriel and San Bernardino Mountains of southern California. These siblings in the Central Transverse Ranges are juxtaposed across the San Andreas fault and have been tectonically uplifted since the late Miocene under roughly similar conditions, yet their geomorphic expressions are very different. Because these ranges share numerous boundary conditions and because their syn-uplift exhumation is constrained by thermochronometric and geologic data, they provide a template to explore the role of specific parameters on the erosion of mountains. To address this, existing constraints on long-term exhumation are synthesized and used to construct best-guess models of erosion patterns in each range. These patterns of erosion are then compared to variables that are generally considered to be important influences on erosion rate. Erosion rate is correlated with topographic slope, suggesting that the two are coupled and that the questions of what controls erosion and what controls topography are interchangeable. Erosion patterns are also strongly influenced by the distribution of active structures, suggesting that deformation alone may be responsible for the observed patterns within each range. However, additional correlations with erosion exist in bedrock erodibility and mean annual precipitation. Altogether, broad differences between the two ranges may be explained by either structure, bedrock erodibility, long-term precipitation, or the greater duration of uplift of the San Gabriel Mountains. All of the boundary conditions that are generally considered important in the erosion of mountains are thus found to be related to erosion patterns in the Central Transverse Ranges. Without a means to isolate the relative influence of each parameters, however, we conclude that geomorphic differences between the ranges are the result of coincidental spatial arrangement of independent variables that are important for sculpting mountains.

Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome

Abstract New U-Pb zircon, monazite, 40Ar/39Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica 40Ar/39Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar 40Ar/39Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar 40Ar/39Ar spectra yield ages of c. 11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below c. 115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from c. 40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to c. 350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica 40Ar/39Ar chrontours are domed, whereas the low-temperature step potassium feldspar 40Ar/39Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.

Low-temperature thermochronology of the San Gabriel and San Bernardino Mountains, southern California: Constraining structural evolution

Recently, apatite fission-track and (U-Th)/He analyses have been used to constrain the low temperature thermal history of the San Gabriel and San Bernardino Mountains of the central Transverse Ranges of southern California. In this paper, we use these data to estimate the timing of initiation of exhumation (uplift and erosion) on fault blocks within both ranges. We calculate average exhumation rates for different time intervals and reconstruct the evolution of both ranges and adjacent fault blocks over the past 12 m.y. using the Matti and Morton (1993) reconstructions as a base. With these reconstructions we are able to evaluate the variations in response (indicated by exhumation rate) to tectonic forces as the San Gabriel Mountains moved past the San Bernardino Mountains. In general, the San Bernardino Mountains have a much simpler exhumational and structural history than the San Gabriel Mountains, with the most recent phase of exhumation and uplift of geomorphic surfaces beginning ca. 2.5 Ma. This time is long after the most recent phases of exhumation began on fault blocks within the San Gabriel Mountains. There, distinct variations in fault-block exhumation patterns began ca. 12 Ma, when the southern strand of the San Gabriel fault became active. We conclude that although some similarities in exhumational history between the two ranges are evident beginning ca. 3-2.5 Ma, the differences in exhumational patterns between the two ranges indicate that they are not reacting similarly to regional tectonic stresses. The exhumational patterns observed suggest that the San Jacinto Mountains-Peninsular Ranges province is acting as an indentor, controlling when faults have turned on and off and the loci of highest exhumation rates.

Active Tectonics and Ultrahigh-Pressure Rocks

This chapter compares modern exhumation and surface uplift rates with the rates needed for the preservation of ultrahigh pressure (UHP) metamorphic rocks. The highest recorded exhumation rates of ~ 5–10 mm/a are inferred from isotopic and fission-track analyses in the Himalaya, Southern Alps of New Zealand, and D’Entrecasteaux Islands. Similar rates (~7 mm/a) of surface uplift are measured from leveling surveys in Nepal and correlations of marine terraces in the Southern Alps. In Nepal, however, this surface uplift rate is occurring despite erosion, and the true rate of surface uplift is probably considerably higher. In restraining bends along the San Andreas and Denali strike-slip faults in North America, contraction has produced localized regions with relatively high exhumation rates of 1–5 mm/a. A similar surface uplift rate (3–5 mm/a) was obtained from marine terrace correlations in the King Range of northern California.

Exhumation of the Southern Sierra Nevada–Eastern Tehachapi Mountains Constrained by Low-Temperature Thermochronology: Implications for the Initiation of the Garlock Fault

New apatite and zircon fission-track and apatite (U-Th)/He data from nine samples collected on a north-south transect across the southern Sierra Nevada–eastern Tehachapi Mountains constrain the cooling and exhumation history over the past ∼70 m.y. The four northernmost samples yielded zircon and apatite fission-track ages of ca. 70 Ma, indicating rapid cooling from ∼250 °C to